Unit 4: Vector Overlay Analysis. Lecture Outline. Possible background reading material: McHarg, Ian 1992 Design with Nature. Wiley and Sons, New York.

Transcription

1 Unit 4: Vector Overlay Analysis Lecture Outline Possible background reading material: McHarg, Ian 1992 Design with Nature. Wiley and Sons, New York. Introduction: The idea of overlay originated when maps were produced using mylar (clear acetate). Mylar maps could be laid on top of one another, giving the user an idea of what areas or other features on a map layer coincided with areas or other features on another map layer. One of the earliest examples of this technique is illustrated in Ian McHarg s Design with Nature (1992), in which McHarg, an environmental planner, used manual overlay of mapped data to show urban and ecological associations. Overlay in the topological vector data model is conceptually the same, except that while new features might be created when 2 GIS layers are used in an overlay spatial operation, the resulting attribute table will be some modification of the attribute tables of the original tables, except in rare cases. Generally, spatial information contained in a study area is divided into different feature classes, primarily based upon the 3 types of spatial entities first (point, line or polygon), and then according to some theme, as seen above. It is usually necessary to combine some or all of these feature class layers in

2 order to answer questions about the study area. For instance, combining a vegetation cover layer with a soils layer in order to determine the amount of area that vegetation cover type A occurs with soil type 1. An important component in vector overlay is whether the GIS layers used in the overlay operation are coincident. Layers that are not aligned properly will introduce additional error in the overlay process. Technically, overlay is a category of functions that puts two layers of the same study area on top of each other inorder to to create new output features. Attributes of each input feature are combined from the two layers that describe each new output feature. More specifically, the order of overlaying the layers can be important because order can affect the output features produced.

3 There are 3 main types of vector overlay operations: union, intersect and identity. A union overlay is equivalent to the Boolean logic: A OR B. Intersect overlay is equivalent to the Boolean logic A AND B.

4 Identity overlay is a combination of union and intersect overlay whose Boolean equivalent is A OR (B AND A). The graphics and tables below show a union operation. Examine the output coverage table and compare it to the output coverage tables for the intersect and identity overlay operations. How many records are in this output table?

5 The graphics and tables below show an intersect overlay operation. Compare the output geometry and attribute table with the preceding union output geometry and attribute table. Why are there fewer records in the output attribute table than when using the union overlay? The graphics and tables below show an identity overlay operation. In what way does the output geometry look different when compared to the output geometry of both the union and intersect overlay operations?

6 Overlay operations are not restricted to polygon layers. Other types of overlay include line-in-polygon and point-in-polygon overlays. Look at the output layer for each to predict what kind of spatial object layer is produced by each type of overlay (point, line, polygon). Be aware, however, that the GIS software you are working with might not support all of these combinations of overlay operations. The following graphic illustrates these types of overlay operations for line-in-polygon and point-inpolygon for the 3 categories of overlay (union, intersect, identity).

7 Line-in-Polygon Intersect Overlay: output layer is a line layer Point-in-polygon intersect overlay: output is a point layer

8 Line-in-polygon identity overlay: output layer is a line layer.